(1) Field of the Invention:
The present invention relates to low expansion ceramics and a process for producing the same. More particularly, the invention relates to heat-resisting, low thermal expansion zirconyl phosphate ceramics having excellent thermal shock resistance and heat resistance, and a process for producing them.
(2) Related Art Statement:
With recent advances in industrial technology, demands for materials having excellent heat resistance and thermal shock resistance have increased. Thermal shock resistance of ceramics is influenced by characteristics of materials such as coefficient of thermal expansion, thermal conductivity, strength, Poisson's ratio, etc. as well as the size and configuration of products and heating and cooling states, that is, heat transfer rate.
Among these factors influencing the thermal shock resistance, the contributory percentage of the coefficient of thermal expansion is particularly great. It is known that particularly when the heat transfer rate is large, the thermal shock resistance greatly depends upon the coefficient of thermal expansion only. Thus, desires to develop low expansion materials having excellent thermal shock resistance are still strong.
As relatively low expansion ceramic materials having a coefficient of thermal expansion of about 5.times.10.sup.-7 to about 20.times.10.sup.-7 (1/.degree. C.) in a temperature range from 40.degree. C. to 800.degree. C., there exist cordierite (MAS), lithium.aluminum.silicate (LAS), etc. Melting points of MAS and LAS are low, that is, 1,450.degree. C. and 1,423.degree. C., respectively. For example, when the fitting location of a catalyst converter is changed from a conventional underbed to near an engine to improve purifying efficiency of a catalyst or fitting of a turbocharger is made to improve fuel consumption and engine output; such design changes raise temperatures of exhaust gases as compared with conventional techniques. Accordingly, with respect to ceramic honeycomb structural bodies used in automobile catalytic purifiers, since the temperature of a catalyst bed is raised by 100.degree. to 200.degree. C., it is found that even honeycomb structural carriers mainly composed of cordierite having a high melting point may be clogged due to melting thereof. Under the circumstances, low expansion materials having thermal shock resistance equivalent to or better than that of cordierite as well as excellent heat resistance have earnestly been demanded.
As ceramics having relatively low thermal expansion and high heat resistance, there are available mullite (3Al.sub.2 O.sub.3.2SiO.sub.2, coefficient of thermal expansion: 53.times.10.sup.-7 /.degree. C., melting point: 1,750.degree. C.) and zircon (ZrO.sub.2.SiO.sub.2, coefficient of thermal expansion: 42.times.10.sup.-7 /.degree. C., melting point: 1,720.degree. C.) only. However, both of them have the shortcoming that the coefficient of thermal expansion is high and thermal shock resistance is low.
Further, as known low expansion ceramics mainly consisting of zirconyl phosphate, Japanese patent publication No. 61-12,867 describes high strength zirconyl phosphate sintered bodies containing 2-10 molar % of a mixture of SiO.sub.2 /Nb.sub.2 O.sub.5 in a molar ratio of 1 to 8 and 1-6 molar % of Al.sub.2 O.sub.3. Japanese patent application laid-open No. 60-21,853 describes low expansion zirconium phosphate porcelain containing 0.5 to 6% by weight of magnesium phosphate as a sintering aid. Japanese patent application laid-open No. 61-219,753 describes a process for producing low expansion zirconyl phosphate ceramics by adding 0.3 to 10% by weight of at least one kind of each of a group consisting of ZrO, MgO, Bi.sub.2 O.sub.3, MnO.sub.2, Co.sub.2 O.sub.3, NiO, TiO.sub.2, CeO.sub.2, Nb.sub.2 O.sub.5, and Ta.sub.2 O.sub.5 as a sintering promoter and a group consisting of SiO.sub.2 and a silicate as a grain growth regulator in totally two kinds. Yougyou Gijyutsu Research Institute annual report No. 9, pp 23 to 30 (1982) of Nagoya Industrial University describes zirconyl phosphate ceramics containing 2% by weight of an additive such as MgO, MnO.sub.2, Fe.sub.2 O.sub.3, or ZnO. Each of such sintered bodies have poor heat resistance due to liquid phase sintering in which a sintering mechanism is based on a formation of a liquid phase at a low melting point, and cannot meet the above-mentioned demands.
These sintered bodies have an open porosity of almost zero, are very dense, and include no microcracks in their fine structure. Thus, their coefficients of thermal expansion are relatively high, so that they have poor thermal shock resistance.